(OPRAH.com) -- Four years ago, when Oprah managed to get down to a trim and fit 160 pounds, she thought she'd hit on a foolproof formula for permanent weight loss. Then life --in the form of a thyroid problem and a killer schedule-- intervened. Last year she was back up to the 200-pound mark and knew something had to change. After a desperately needed time-out to reflect and recharge, here's what she's learned, what she's doing differently, and what's next.

Oprah Winfrey attends a gala on December 3, 2008 in New York City.

You know how bad you feel when you have a special event, a reunion, a wedding, a bar mitzvah, and you wanted to lose that extra 10 to 40 pounds, and you didn't do it?

So the day comes and now you've got to try to find something to wear that makes you feel halfway decent, and you have to figure out how to hold in your stomach all night and walk backward out of the room so no one sees that your butt keeps moving even when you stop. Multiply that feeling by a million -- make that more than 2.4 million for every O reader -- and you'll know how I've felt over the past year every time I had to shoot a cover for O. If you're a regular subscriber, you'll notice you've not seen a head-to-toe shot all year. Why? Because I didn't want to be seen. "

In 1992 I reached my heaviest, 237 pounds. I was 38. Then, four years ago, I made it a goal to lose weight, and I appeared on the January 2005 cover at a toned 160 pounds. I thought I was finished with the weight battle. I was done. I'd conquered it. I was so sure, I was even cocky. I had the nerve to say to friends who were struggling, "All you have to do is work out harder and eat less! Get your 10,000 steps in! None of that starchy stuff!"

Bam! Karma is a bear of a thing.

So here I stand, 40 pounds heavier than I was in 2006. (Yes, you're adding correctly; that means the dreaded 2-0-0.) I'm mad at myself. I'm embarrassed. I can't believe that after all these years, all the things I know how to do, I'm still talking about my weight. I look at my thinner self and think, "How did I let this happen again?" Oprah.com: How to prevent weight gain relapse

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It happened slowly. In February 2007, at 53, I started to have some health issues. At first I was unable to sleep for days. My legs started swelling. My weight started creeping up, first 5 pounds, then 10 pounds. I was lethargic and irritable. My internal clock seemed totally out of whack. I began having rushing heart palpitations every time I worked out. Okay, I've never loved daily exercise, but this was different. I actually developed a fear of working out. I was scared that I would pass out. Or worse. I felt as if I didn't know my own body anymore.

After many trips to various doctors, I received a diagnosis. I had hyperthyroidism (an overactive thyroid that can speed up metabolism and cause weight loss -- but of course didn't make me lose a single pound) and then gradually started moving into hypothyroidism (a sluggish metabolism that can cause fatigue and weight gain). My doctor prescribed medication and warned me that I must "learn to embrace hunger" or I would immediately gain weight. Believe me, no part of me was prepared to embrace hunger.

It seemed as if the struggle I'd had with weight my entire adult life was now officially over. I felt completely defeated. I thought, "I give up. I give up. Fat wins." All these years I'd had only myself to blame for lack of willpower. Now I had an official, documented excuse.

The thyroid diagnosis felt like some kind of prison sentence. I was so frustrated that I started eating whatever I wanted -- and that's never good. My drug of choice is food. I use food for the same reasons an addict uses drugs: to comfort, to soothe, to ease stress.

I switched doctors and still gained weight. At one point I was on three medications: one for heart palpitations, another for high blood pressure, another to moderate my thyroid. Who knew this tiny butterfly gland at the base of the throat had so much power? When it's off, your whole body feels the effects. I followed my doctor's orders to the letter (except for the part about working out). I took the prescribed medication religiously at the same time each day. Oprah.com: The truth about your thyroid

Being medicated, though necessary, made me feel as if I were viewing life through a veil. I felt like an invalid. Everything was duller. I felt like the volume on life got turned down.

I realized this to some extent, but I wasn't fully aware of the effect of the medication until I had a conversation with my friend Bob Greene. He'd given up lecturing me about working out and eating well, but we were walking together one day and he said, "I think something's wrong. You're listless. Your movements are slower, even when you're just doing normal stuff. Twice I've told you something and you don't remember it. There's no sparkle in your eyes. I think you're in some sort of depression." Greene talks about Oprah's struggle »

Me -- depressed? I hadn't thought I was, but definitely something was off. I felt like the life force was being sucked out of me. I always had an excuse for being tired. It took extra effort to do everything. I didn't want to go anywhere, and I didn't want to be seen any more than I had to. I could oversee a show and a magazine that tell people how to live their best lives, but I definitely wasn't setting an example. I was talking the talk, but I wasn't walking the walk. And that was very disappointing to me.

Immediately after that conversation with Bob, I called my doctor. "All this medicine is making my life feel like a flat line," I said. So my doctor slowly weaned me off it, except for one aspirin a day. (By the way, never suddenly stop taking prescribed medication, especially heart and blood pressure medication, without checking with your physician.) That choice was the beginning of my road back to health -- and back to myself. Oprah.com: Oprah's Best Life Week begins January 5